CI builders for all versions mentioned in the build script

This should prevent accidentally inserting something under one of these
cfgs that is available only on a newer rustc. For example if something
is changed in the Duration serialization, but that change works only on
a recent rustc, our test suite will not have caught it before.

.travis.yml

@@ -4,11 +4,15 @@ cache: cargo
 
 # run builds for all the trains (and more)
 rust:
-  - 1.13.0
-  - 1.15.0
   - stable
   - beta
   - nightly
+  - 1.13.0
+  - 1.15.0
+  - 1.20.0
+  - 1.21.0
+  - 1.25.0
+  - 1.26.0
 
 matrix:
   include:

travis.sh

@@ -93,4 +93,12 @@ else
     cargo clean
     cd "$DIR/serde_derive"
     channel build
+
+    for CHANNEL in 1.20.0 1.21.0 1.25.0 1.26.0; do
+        cd "$DIR"
+        cargo clean
+        cd "$DIR/serde"
+        channel build --no-default-features
+        channel build
+    done
 fi